Thin films of codeposited cadmium/selenium for solar cells have been prepared by means of electrodeposition from aqueous solutions containing cadmium ions and selenious acid. U.S. Pat. No. 4,253,919 discloses this technique where hydrated citric acid is used to facilitate the codeposition of selenium and cadmium on the substrate. It was found that the use of citrate baths avoided the need of complexing agents to retard deposition of the more easily deposited species of the cadmium/selenium elements. Further example of this technique of electrodepositing the thin film of cadmium selenide from aqueous solutions containing cadmium ions and selenious acid is disclosed in M. Skyllas-Kayacos and B. Miller, J. Electrochem. Soc. 127, 869 (1980) and M. Tomkiewicz, I. Ling and W. S. Parsons, J. Electrochem. Soc. 129, 2016.
The difficulty in electrodepositing thin films of cadmium/selenium from the solutions containing selenious acid is that a substantial excess of selenium is detected in the films. To remove a major portion of this excess, the films must be annealed at temperatures as high as 750.degree. C. It is thought that the selenium incorporation occurs according to the reaction EQU 2CdSe+H.sub.2 SeO.sub.3 +4H.sup.+ .fwdarw.2Cd.sup.2+ +3Se+3H.sub.2 O
It was, therefore, important to avoid having this reaction occur in the electrolyte bath.
The selenosulphite ion has been employed in forming cadmium selenide films by a chemical deposition process as disclosed in R. A. Boudreau and R. D. Rauh, J. Electrochem. Soc. 130, 513 (1983). The process involves the precipitation of cadmium selenide from a solution at 90.degree. C. containing a cadmium-amine complex and sodium selenosulphite. This proves to be complicated, difficult to control and non-specific regarding the areas of deposition.
An alternate approach to the electrodeposition of cadmium selenide involves the use of cyanide solutions as disclosed in M. Skyllas-Kazacos, J. Electroanal. Chem. 148, 233, (1983). The cadmium ions are complexed with EDTA and a selenocyanide ion is incorporated in the solution which is reduced at the cathode to produce selenide ions which react with the cadmium ions to electrodeposit cadmium selenide on the cathode. However, the solar energy conversion efficiencies of films produced by this process are low.
The process, according to this invention, is capable of electrodepositing relatively large surface area thin films of cadmium selenide at room temperature without any requirement to control the atmosphere about the system.